Network Working Group G. Camarillo, Ed.
Request for Comments: 3312 Ericsson
Category: Standards Track W. Marshall, Ed.
AT&T
J. Rosenberg
dynamicsoft
October 2002
Integration of Resource Managementand Session Initiation Protocol (SIP)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines a generic framework for preconditions, which
are extensible through IANA registration. This document also
discusses how network quality of service can be made a precondition
for establishment of sessions initiated by the Session Initiation
Protocol (SIP). These preconditions require that the participant
reserve network resources before continuing with the session. We do
not define new quality of service reservation mechanisms; these
preconditions simply require a participant to use existing resource
reservation mechanisms before beginning the session.
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RFC 3312 Integration of Resource Management and SIP October 2002
The solution is to introduce the concept of a precondition. A
precondition is a set of constraints about the session which are
introduced in the offer. The recipient of the offer generates an
answer, but does not alert the user or otherwise proceed with session
establishment. That only occurs when the preconditions are met.
This can be known through a local event (such as a confirmation of a
resource reservation), or through a new offer sent by the caller.
This document deals with sessions that use SIP [1] as a signalling
protocol and SDP [2] to describe the parameters of the session.
We have chosen to include the quality of service preconditions in the
SDP description rather than in the SIP header because preconditions
are stream specific.
2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [3].
3 Overview
In order to ensure that session establishment does not take place
until certain preconditions are met, we distinguish between two
different state variables that affect a particular media stream:
current status and desired status. This document defines the quality
of service status.
The desired status consists of a threshold for the current status.
Session establishment stops until the current status reaches or
surpasses this threshold. Once this threshold is reached or
surpassed, session establishment resumes.
For example, the following values for current and desired status
would not allow session establishment to resume:
current status = resources reserved in the send direction
desired status = resources reserved in both (sendrecv) directions
On the other hand, the values of the example below would make session
establishment resume:
current status = resources reserved in both (sendrecv) directions
desired status = resources reserved in the send direction
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RFC 3312 Integration of Resource Management and SIP October 2002
These two state variables define a certain piece of state of a media
stream the same way the direction attribute or the codecs in use
define other pieces of state. Consequently, we treat these two new
variables in the same way as other SDP media attributes are treated
in the offer/answer model used by SIP [4]: they are exchanged between
two user agents using an offer and an answer in order to have a
shared view of the status of the session.
Figure 1 shows a typical message exchange between two SIP user agents
using preconditions. A includes quality of service preconditions in
the SDP of the initial INVITE. A does not want B to be alerted until
there are network resources reserved in both directions (sendrecv)
end-to-end. B agrees to reserve network resources for this session
before alerting the callee. B will handle resource reservation in
the B->A direction, but needs A to handle the A->B direction. To
indicate so, B returns a 183 (Session Progress) response to A asking
A to start resource reservation and to confirm to B as soon as the
A->B direction is ready for the session. A and B both start resource
reservation. B finishes reserving resources in the B->A direction,
but does not alert the user yet, because network resources in both
directions are needed. When A finishes reserving resources in the
A->B direction, it sends an UPDATE [5] to B. B returns a 200 (OK)
response for the UPDATE, indicating that all the preconditions for
the session have been met. At this point in time, B starts alerting
the user, and session establishment completes normally.
4 SDP parameters
We define the following media level SDP attributes:
current-status = "a=curr:" precondition-type
SP status-type SP direction-tag
desired-status = "a=des:" precondition-type
SP strength-tag SP status-type
SP direction-tag
confirm-status = "a=conf:" precondition-type
SP status-type SP direction-tag
precondition-type = "qos" | token
strength-tag = ("mandatory" | "optional" | "none"
= | "failure" | "unknown")
status-type = ("e2e" | "local" | "remote")
direction-tag = ("none" | "send" | "recv" | "sendrecv")
Current status: The current status attribute carries the current
status of network resources for a particular media stream.
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RFC 3312 Integration of Resource Management and SIP October 2002
Desired status: The desired status attribute carries the
preconditions for a particular media stream. When the
direction-tag of the current status attribute, with a given
precondition-type/status-type for a particular stream is
equal to (or better than) the direction-tag of the desired
status attribute with the same precondition-type/status-
type, for that stream, then the preconditions are considered
to be met for that stream.
Confirmation status: The confirmation status attribute carries
threshold conditions for a media stream. When the status of
network resources reach these conditions, the peer user
agent will send an update of the session description
containing an updated current status attribute for this
particular media stream.
Precondition type: This document defines quality of service
preconditions. Extensions may define other types of
preconditions.
Strength tag: The strength-tag indicates whether or not the callee
can be alerted, in case the network fails to meet the
preconditions.
Status type: We define two types of status: end-to-end and
segmented. The end-to-end status reflects the status of the
end-to-end reservation of resources. The segmented status
reflects the status of the access network reservations of
both user agents. The end-to-end status corresponds to the
tag "e2e", defined above and the segmented status to the
tags "local" and "remote". End-to-end status is useful when
end-to-end resource reservation mechanisms are available.
The segmented status is useful when one or both UAs perform
resource reservations on their respective access networks.
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RFC 3312 Integration of Resource Management and SIP October 2002
The values of the tags "send", "recv", "local" and "remote" represent
the point of view of the entity generating the SDP description. In
an offer, "send" is the direction offerer->answerer and "local" is
the offerer's access network. In an answer, "send" is the direction
answerer->offerer and "local" is the answerer's access network.
The following example shows these new SDP attributes in two media
lines of a session description:
m=audio 20000 RTP/AVP 0
a=curr:qos e2e send
a=des:qos optional e2e send
a=des:qos mandatory e2e recv
m=audio 20002 RTP/AVP 0
a=curr:qos local sendrecv
a=curr:qos remote none
a=des:qos optional local sendrecv
a=des:qos mandatory remote sendrecv
5 Usage of preconditions with offer/answer
Parameter negotiation in SIP is carried out using the offer/answer
model described in [4]. The idea behind this model is to provide a
shared view of the session parameters for both user agents once the
answer has been received by the offerer. This section describes
which values our new SDP attributes can take in an answer, depending
on their value in the offer.
To achieve a shared view of the status of a media stream, we define a
model that consists of three tables: both user agents implement a
local status table, and each offer/answer exchange has a transaction
status table associated to it. The offerer generates a transaction
status table, identical to its local status table, and sends it to
the answerer in the offer. The answerer uses the information of this
transaction status table to update its local status table. The
answerer also updates the transaction status table fields that were
out of date and returns this table to the offerer in the answer. The
offerer can then update its local status table with the information
received in the answer. After this offer/answer exchange, the local
status tables of both user agents are synchronised. They now have a
common view of the status of the media stream. Sessions that involve
several media streams implement these tables per media stream. Note,
however, that this is a model of user agent behavior, not of
software. An implementation is free to take any approach that
replicates the external behavior this model defines.
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RFC 3312 Integration of Resource Management and SIP October 20025.1 Generating an offer
Both user agents MUST maintain a local precondition status, which is
referred to as a "local status table". Tables 1 and 2 show the
format of these tables for both the end-to-end and the segmented
status types. For the end-to-end status type, the table contains two
rows; one for each direction (i.e., send and recv). A value of "yes"
in the "Current" field indicates the successful reservation of that
resource in the corresponding direction. "No" indicates that
resources have not been reserved yet. The "Desired Strength" field
indicates the strength of the preconditions in the corresponding
direction. The table for the segmented status type contains four
rows: both directions in the local access network and in the peer's
access network. The meaning of the fields is the same as in the
end-to-end case.
Before generating an offer, the offerer MUST build a transaction
status table with the current and the desired status, for each media
stream. The different values of the strength-tag for the desired
status attribute have the following semantics:
o None: no resource reservation is needed.
o Optional: the user agents SHOULD try to provide resource
reservation, but the session can continue regardless of whether
or not this provision is possible.
o Mandatory: the user agents MUST provide resource reservation.
Otherwise, session establishment MUST NOT continue.
The offerer then decides whether it is going to use the end-to-end
status type or the segmented status type. If the status type of the
media line will be end-to-end, the user agent generates records with
the desired status and the current status for each direction (send
and recv) independently, as shown in table 1:
Direction Current Desired Strength
____________________________________
send no mandatory
recv no mandatory
Table 1: Table for the end-to-end status type
If the status type of the media line will be segmented, the user
agent generates records with the desired status and the current
status for each direction (send and recv) and each segment (local and
remote) independently, as shown in table 2:
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RFC 3312 Integration of Resource Management and SIP October 2002
Direction Current Desired Strength
______________________________________
local send no none
local recv no none
remote send no optional
remote recv no none
Table 2: Table for the segmented status type
At the time of sending the offer, the offerer's local status table
and the transaction status table contain the same values.
With the transaction status table, the user agent MUST generate the
current-status and the desired status lines, following the syntax of
Section 4 and the rules described below in Section 5.1.1.
5.1.1 SDP encoding
For the end-to-end status type, the user agent MUST generate one
current status line with the tag "e2e" for the media stream. If the
strength-tags for both directions are equal (e.g., both "mandatory")
in the transaction status table, the user agent MUST add one desired
status line with the tag "sendrecv". If both tags are different, the
user agent MUST include two desired status lines, one with the tag
"send" and the other with the tag "recv".
The semantics of two lines with the same strength-tag, one with a
"send" tag and the other with a "recv" tag, is the same as one
"sendrecv" line. However, in order to achieve a more compact
encoding, we have chosen to make the latter format mandatory.
For the segmented status type, the user agent MUST generate two
current status lines: one with the tag "local" and the other with the
tag "remote". The user agent MUST add one or two desired status
lines per segment (i.e., local and remote). If, for a particular
segment (local or remote), the tags for both directions in the
transaction status table are equal (e.g., both "mandatory"), the user
agent MUST add one desired status line with the tag "sendrecv". If
both tags are different, the user agent MUST include two desired
status lines, one with the tag "send" and the other with the tag
"recv".
Note that the rules above apply to the desired strength-tag "none" as
well. This way, a user agent that supports quality of service but
does not intend to use them, adds desired status lines with the
strength-tag "none". Since this tag can be upgraded in the answer,
as described in Section 5.2, the answerer can request quality of
service reservation without a need of another offer/answer exchange.
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RFC 3312 Integration of Resource Management and SIP October 2002
The example below shows the SDP corresponding to tables 1 and 2.
m=audio 20000 RTP/AVP 0
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv
m=audio 20002 RTP/AVP 0
a=curr:qos local none
a=curr:qos remote none
a=des:qos optional remote send
a=des:qos none remote recv
a=des:qos none local sendrecv
5.2 Generating an Answer
When the answerer receives the offer, it recreates the transaction
status table using the SDP attributes contained in the offer. The
answerer updates both its local status and the transaction status
table following the rules below:
Desired Strength: We define an absolute ordering for the
strength-tags: "none", "optional" and "mandatory".
"Mandatory" is the tag with the highest grade and "none" the
tag with the lowest grade. An answerer MAY upgrade the
desired strength in any entry of the transaction status
table, but it MUST NOT downgrade it. Therefore, it is OK to
upgrade a row from "none" to "optional", from "none" to
"mandatory", or from "optional" to "mandatory", but not the
other way around.
Current Status: For every row, the value of the "Current" field in
the transaction status table, and in the local status table
of the answerer, have to be compared. Table 3 shows the
four possible combinations. If both fields have the same
value (two first rows of table 3), nothing needs to be
updated. If the "Current" field of the transaction status
table is "Yes", and the field of the local status table is
"No" (third row of table 3), the latter MUST be set to
"Yes". If the "Current" field of the transaction status
table is "No", and the field of the local status table is
"Yes" (forth row of table 3), the answerer needs to check if
it has local information (e.g., a confirmation of a resource
reservation has been received) about that particular current
status. If it does, the "Current" field of the transaction
status table is set to "Yes". If the answerer does not have
local information about that current status, the "Current"
field of the local status table MUST be set to "No".
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RFC 3312 Integration of Resource Management and SIP October 2002
Transac. status table Local status table New values transac./local
____________________________________________________________________
no no no/no
yes yes yes/yes
yes no yes/yes
no yes depends on local info
Table 3: Possible values for the "Current" fields
Once both tables have been updated, an answer MUST be generated
following the rules described in Section 5.1.1, taking into account
that "send", "recv", "local" and "remote" tags have to be inverted in
the answer, as shown in table 4.
Offer Answer
______________
send recv
recv send
local remote
remote local
Table 4: Values of tags in offers and answers
At the time the answer is sent, the transaction status table and the
answerer's local status table contain the same values. Therefore,
this answer contains the shared view of the status of the media line
in the current-status attribute and the negotiated strength and
direction-tags in the desired-status attribute.
If the resource reservation mechanism used requires participation of
both user agents, the answerer SHOULD start resource reservation
after having sent the answer and the offerer SHOULD start resource
reservation as soon as the answer is received. If participation of
the peer user agent is not needed (e.g., segmented status type), the
offerer MAY start resource reservation before sending the offer and
the answerer MAY start it before sending the answer.
The status of the resource reservation of a media line can change
between two consecutive offer/answer exchanges. Therefore, both user
agents MUST keep their local status tables up to date, using local
information throughout the duration of the session.
6 Suspending and Resuming Session Establishment
A user agent server that receives an offer with preconditions SHOULD
NOT alert the user until all the mandatory preconditions are met;
session establishment is suspended until that moment (e.g., a PSTN
gateway reserves resources without sending signalling to the PSTN.)
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RFC 3312 Integration of Resource Management and SIP October 2002
A user agent server may receive an INVITE request with no offer in
it. In this case, following normal procedures defined in [1] and
[5], the user agent server will provide an offer in a reliable 1xx
response. The user agent client will send the answer in another SIP
request (i.e., the PRACK for the 1xx). If the offer and the answer
contain preconditions, the user agent server SHOULD NOT alert the
user until all the mandatory preconditions in the answer are met.
Note that in this case, a user agent server providing an
initial offer with preconditions, a 180 (Ringing) response with
preconditions will never be sent, since the user agent server
cannot alert the user until all the preconditions are met.
A UAS that is not capable of unilaterally meeting all of the
mandatory preconditions MUST include a confirm-status attribute in
the SDP (offer or answer) that it sends (see Section 7). Further,
the SDP (offer or answer) that contains this confirm-status attribute
MUST be sent as soon as allowed by the SIP offer/answer rules.
While session establishment is suspended, user agents SHOULD not send
any data over any media stream. In the case of RTP [6], neither RTP
nor RTCP packets are sent.
A user agent server knows that all the preconditions are met for a
media line when its local status table has a value of "yes" in all
the rows whose strength-tag is "mandatory". When the preconditions
of all the media lines of the session are met, session establishment
SHOULD resume.
For an initial INVITE, suspending and resuming session establishment
is very intuitive. The callee will not be alerted until all the
mandatory preconditions are met. However, offers containing
preconditions sent in the middle of an ongoing session need further
explanation. Both user agents SHOULD continue using the old session
parameters until all the mandatory preconditions are met. At that
moment, the user agents can begin using the new session parameters.
Section 13 contains an example of this situation.
7 Status Confirmation
The confirm-status attribute MAY be used in both offers and answers.
This attribute represents a threshold for the resource reservation.
When this threshold is reached or surpassed, the user agent MUST send
an offer to the peer user agent, reflecting the new current status of
the media line as soon as allowed by the SIP offer/answer rules. If
this threshold is crossed again (e.g., the network stops providing
resources for the media stream), the user agent MUST send a new offer
as well, as soon as allowed by the SIP offer/answer rules.
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RFC 3312 Integration of Resource Management and SIP October 2002
If a peer has requested confirmation on a particular stream, an agent
MUST mark that stream with a flag in its local status table. When
all the rows with this flag have a "Current" value of "yes", the user
agent MUST send a new offer to the peer. This offer will contain the
current status of resource reservation in the current-status
attributes. Later, if any of the rows with this flag transition to
"No", a new offer MUST be sent as well.
Confirmation attributes are not negotiated. The answerer uses the
value of the confirm-status attribute in the offer, and the offerer
uses the value of this attribute in the answer.
For example, if a user agent receives an SDP description with the
following attributes:
m=audio 20002 RTP/AVP 0
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
a=conf:qos remote sendrecv
It will send an offer as soon as it reserves resources in its access
network ("remote" tag in the received message) for both directions
(sendrecv).
8 Refusing an offer
We define a new SIP status code:
Server-Error = "580" ;Precondition Failure
When a UAS, acting as an answerer, cannot or is not willing to meet
the preconditions in the offer, it SHOULD reject the offer by
returning a 580 (Precondition-Failure) response.
Using the 580 (Precondition Failure) status code to refuse an offer
is useful when the offer comes in an INVITE or in an UPDATE request.
However, SIP does not provide a means to refuse offers that arrive in
a response (1xx or 2xx) to an INVITE. If a UAC generates an initial
INVITE without an offer and receives an offer in a 1xx or 2xx
response which is not acceptable, it SHOULD respond to this offer
with a correctly formed answer and immediately send a CANCEL or a
BYE.
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RFC 3312 Integration of Resource Management and SIP October 2002
If the offer comes in a 1xx or 2xx response to a re-INVITE, A would
not have a way to reject it without terminating the session at the
same time. The same recommendation given in Section 15.2 of [1]
applies here:
"The UAS MUST ensure that the session description overlaps with
its previous session description in media formats, transports,
other parameters that require support from the peer. This is
to avoid the need for the peer to reject the session
description. If, however, it is unacceptable to A, A SHOULD
generate an answer with a valid session description, and then
send a BYE to terminate the session."
580 (Precondition Failure) responses and BYE and CANCEL requests,
indicating failure to meet certain preconditions, SHOULD contain an
SDP description, indicating which desired status triggered the
failure. Note that this SDP description is not an offer or an
answer, since it does not lead to the establishment of a session.
The format of such a description is based on the last SDP (an offer
or an answer) received from the remote UA.
For each "m=" line in the last SDP description received, there MUST
be a corresponding "m=" line in the SDP description indicating
failure. This SDP description MUST contain exactly the same number
of "m=" lines as the last SDP description received. The port number
of every "m=" line MUST be set to zero, but the connection address is
arbitrary.
The desired status line corresponding to the precondition that
triggered the failure MUST use the "failure" strength-tag, as shown
in the example below:
m=audio 20000 RTP/AVP 0
a=des:qos failure e2e send
8.1 Rejecting a Media Stream
In the offer/answer model, when an answerer wishes to reject a media
stream, it sets its port to zero. The presence of preconditions does
not change this behaviour; streams are still rejected by setting
their port to zero.
Both the offerer and the answerer MUST ignore all the preconditions
that affect a stream with its port set to zero. They are not taken
into consideration to decide whether or not session establishment can
resume.
Camarillo, et. al. Standards Track [Page 14]

RFC 3312 Integration of Resource Management and SIP October 20029 Unknown Precondition Type
This document defines the "qos" tag for quality of service
preconditions. New precondition-types defined in the future will
have new associated tags. A UA that receives an unknown
precondition-type, with a "mandatory" strength-tag in an offer, MUST
refuse the offer unless the only unknown mandatory preconditions have
the "local" tag. In this case, the UA does not need to be involved
in order to meet the preconditions. The UA will ask for confirmation
of the preconditions and, when the confirmation arrives, it will
resume session establishment.
A UA refusing an offer follows the rules described in section 8, but
instead of the tag "failure", it uses the tag "unknown", as shown in
the example below:
m=audio 20000 RTP/AVP 0
a=des:foo unknown e2e send
10 Multiple Preconditions per Media Stream
A media stream MAY contain multiple preconditions. Different
preconditions MAY have the same precondition-type and different
status-types (e.g., end to end and segmented quality of service
preconditions) or different precondition-types (this document only
defines the "qos" precondition type, but extensions may define more
precondition-types in the future).
All the preconditions for a media stream MUST be met in order to
resume session establishment. The following example shows a session
description that uses both end-to-end and segmented status-types for
a media stream.
m=audio 20000 RTP/AVP 0
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
a=curr:qos e2e none
a=des:qos optional e2e sendrecv
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RFC 3312 Integration of Resource Management and SIP October 200211 Option Tag for Preconditions
We define the option tag "precondition" for use in the Require and
Supported header fields. An offerer MUST include this tag in the
Require header field if the offer contains one or more "mandatory"
strength-tags. If all the strength-tags in the description are
"optional" or "none", the offerer MUST include this tag in either a
Supported header field or in a Require header field. It is, however,
RECOMMENDED that the Supported header field be used in this case.
The lack of preconditions in the answer would indicate that the
answerer did not support this extension.
The mapping of offers and answers to SIP requests and responses is
performed following the rules given in [5]. Therefore, a user agent
including preconditions in the SDP MUST support the PRACK and UPDATE
methods. Consequently, it MUST include the "100rel" [7] tag in the
Supported header field and SHOULD include an Allow header field with
the "UPDATE" tag [5].
12 Indicating Capabilities
The offer/answer model [4] describes the format of a session
description to indicate capabilities. This format is used in
responses to OPTIONS requests. A UA that supports preconditions
SHOULD add desired status lines indicating the precondition-types
supported for each media stream. These lines MUST have the "none"
strength-tag, as shown in the example below:
m=audio 0 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=des:foo none e2e sendrecv
a=des:qos none local sendrecv
Note that when this document was published, the precondition-type
"foo" has not been registered. It is used here in the session
description above to provide an example with multiple precondition-
types.
A UA that supports this framework SHOULD add a "precondition" tag to
the Supported header field of its responses to OPTIONS requests.
13 Examples
The following examples cover both status types; end-to-end and
segmented.
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RFC 3312 Integration of Resource Management and SIP October 200213.1 End-to-end Status Type
The call flow of Figure 2 shows a basic session establishment using
the end-to-end status type. The SDP descriptions of this example are
shown below:
SDP1: A includes end-to-end quality of service preconditions in the
initial offer.
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv
SDP2: Since B uses RSVP, it can know when resources in its "send"
direction are available, because it will receive RESV messages from
the network. However, it does not know the status of the
reservations in the other direction. B requests confirmation for
resource reservations in its "recv" direction to the peer user agent
A in its answer.
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv
a=conf:qos e2e recv
After having sent the answer, B starts reserving network resources
for the media stream. When A receives this answer (2), it starts
performing resource reservation as well. Both UAs use RSVP, so A
sends PATH messages towards B and B sends PATH messages towards A.
As time passes, B receives RESV messages confirming the reservation.
However, B waits until resources in the other direction are reserved
as well, since it did not receive any confirmation and the
preconditions still have not been met.
SDP3: When A receives RESV messages, it sends an updated offer (5) to
B:
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=curr:qos e2e send
a=des:qos mandatory e2e sendrecv
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RFC 3312 Integration of Resource Management and SIP October 2002
SDP1: A includes local and remote QoS preconditions in the initial
offer. Before sending the initial offer, A reserves resources in its
access network. This is indicated in the local current status of the
SDP below:
m=audio 20000 RTP/AVP 0 8
c=IN IP4 192.0.2.1
a=curr:qos local sendrecv
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
SDP2: B reserves resources in its access network and, since all the
preconditions are met, returns an answer in a 180 (Ringing) response
(3).
m=audio 30000 RTP/AVP 0 8
c=IN IP4 192.0.2.4
a=curr:qos local sendrecv
a=curr:qos remote sendrecv
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
Let's assume that after receiving this response, A decides that it
wants to use only PCM u-law (payload 0), as opposed to both PCM u-law
and A-law (payload 8). It would send an UPDATE to B, possibly before
receiving the 200 (OK) for the INVITE (5). The SDP would look like:
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=curr:qos local sendrecv
a=curr:qos remote sendrecv
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
B would generate an answer for this offer and place it in the 200
(OK) for the UPDATE.
Note that this last offer/answer to reduce the number of supported
codecs may arrive to the user agent server after the 200 (OK)
response has been generated. This would mean that the session is
established before A has reduced the number of supported codecs. To
avoid this situation, the user agent client could wait for the first
answer from the user agent before setting its local current status to
"sendrecv".
Camarillo, et. al. Standards Track [Page 22]

RFC 3312 Integration of Resource Management and SIP October 200213.3 Offer in a SIP response
The call flow of Figure 5 shows a basic session establishment where
the initial offer appears in a reliable 1xx response. This example
uses the end-to-end status type. The SDP descriptions of this
example are shown below:
The first INVITE (1) does not contain a session description.
Therefore, the initial offer is sent by B in a reliable 183 (Session
Progress) response.
SDP1: B includes end-to-end quality of service preconditions in the
initial offer. Since B uses RSVP, it can know when resources in its
"send" direction are available, because it will receive RESV messages
from the network. However, it does not know the status of the
reservations in the other direction. B requests confirmation for
resource reservations in its "recv" direction, to the peer user agent
A, in its answer.
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv
a=conf:qos e2e recv
SDP2: A includes its answer in the PRACK for the 183 (Session
Progress) response.
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=curr:qos e2e none
a=des:qos mandatory e2e sendrecv
Camarillo, et. al. Standards Track [Page 23]

RFC 3312 Integration of Resource Management and SIP October 2002
SDP3: When A receives RESV messages, it sends an updated offer (5) to
B:
m=audio 20000 RTP/AVP 0
c=IN IP4 192.0.2.1
a=curr:qos e2e send
a=des:qos mandatory e2e sendrecv
SDP4: B responds with an answer (6) which contains the current status
of the resource reservation (i.e., recv):
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.4
a=curr:qos e2e recv
a=des:qos mandatory e2e sendrecv
As time passes, B receives RESV messages confirming the reservation.
At this point in time, session establishment resumes and B returns a
180 (Ringing) response (7).
14 Security Considerations
An entity in the middle of two user agents establishing a session may
add desired-status attributes making session establishment
impossible. It could also modify the content of the current-status
parameters so that the session is established without meeting the
preconditions. Integrity protection can be used to avoid these
attacks.
An entity performing resource reservations upon reception of
unauthenticated requests carrying preconditions can be an easy target
for a denial of service attack. Requests with preconditions SHOULD
be authenticated.
15 IANA Considerations
This document defines three media level SDP attributes: desired-
status, current-status and conf-status. Their format is defined in
Section 4.
This document defines a framework for using preconditions with SIP.
Precondition-types to be used with this framework are registered by
the IANA when they are published in standards track RFCs. The IANA
Considerations section of the RFC MUST include the following
information, which appears in the IANA registry along with the RFC
number of the publication.
Camarillo, et. al. Standards Track [Page 26]

RFC 3312 Integration of Resource Management and SIP October 200221 Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Camarillo, et. al. Standards Track [Page 30]